1. Field of the Invention
The present invention relates to a method of, and apparatus for, processing low power pseudo-random code sequence signals or pseudo-random data sequences and has particular, but not exclusive, application to telecommunications systems such as cellular telephone systems, hand held digital signal processing apparatus and digital paging systems having an answer back capability.
2. Description of the Related Art
An advantage of transmitting signals as spread spectrum signals or pseudo-random code sequences is that several signals can be transmitted contemporaneously on a single carrier frequency and each of the signals can be recovered by techniques, such as correlation, which involves comparing or multiplying each member of a set of code sequences with the received signals and by a process of trial and error to despread each of the signals which is detected using techniques such as Fourier Analysis.
A particular, but not exclusive, application of the present invention is answer back message paging systems, for example of a type disclosed in PCT Patent Specification WO96/14716, corresponding to U.S. Pat. No. 5,872,522 which is owned in common with the present application, in which primary stations transmit messages to secondary stations which transmit registration requests and/or simple replies to the messages as pseudo-random code sequences. At the primary station, the received spread spectrum signals having strengths lying within an acceptable tolerance range can be processed as a group. The processing requires despreading the signals by matching a respective code sequence from a large set of code sequences with the received signals and detecting the registration and/or reply signal for example by determining the choice of code sequence used or applying Fourier Transform techniques. Processing signals in two stages is computationally intensive. The number of Fourier Transforms to be done limits the system capacity. In order to process say 1000 pseudo random code sequences this will either mean that there is a time penalty which limits the capacity of a system or require more processor power if the time penalty is to be reduced. A system operator is in consequence faced with higher operating costs due to the relatively long time spent in processing each pseudo random code sequence or additional installation and operating costs associated with a more powerful processor in order to process the pseudo-random code sequences quicker.